Cadmium sulfide and lead chromate are among the most commonly used yellow pigments. Unfortunately these pigments are toxic and thus there is a need for alternative yellow pigments. In the search for such alternative yellow pigments, interest has focused on the use of the bismuth-vanadate pigments. U.S. Pat. 4,026,722 discloses the use of bismuth-vanadate pigments represented by the formula EQU BiVO.sub.4.xAl.sub.2 O.sub.3.ySiO.sub.2
wherein x is about 0.25-2.0, y is about 0.1-3.5, and the sum of x and y is equal to or greater than 1. U.S. Pat. No. 4,063,956 discloses the use of monoclinic bismuth vanadate pigment containing a precoat of one or more porous hydrous oxides (e.g., aluminum, silicon, titanium, etc.) and a dense overcoat of amorphous silica.
U.S. Pat. Nos. 4,115,141 and 4,115,142 describe the use of monoclinic bismuth vanadate as a yellow pigment for coloring plastics and paints. These pigments are prepared by reacting bismuth nitrate with an alkali vanadate to obtain a bismuth-vanadate gel and then subjecting the gel to aqueous digestion or calcination to produce the bismuth-vanadate pigment.
U.S. Pat. No. 4,230,500 discloses greenish-yellow, yellow and orange-yellow pigments which consist substantially of bismuth vanadate of monoclinic structure, bismuth phosphate of monoclinic structure and aluminum phosphate of orthorhombic structure and which, in the case of yellow and orange-yellow pigments also comprise a compound derived from Bi.sub.2 O.sub.3 and V.sub.2 O.sub.5.
U.S. Pat. No. 4,251,283 discloses greenish-yellow pigments based on BiVO.sub.4 made by the calcination, in the presence of air, of a mixture of BiPO.sub.4, V.sub.2 O.sub.5 and an oxide of Ca, Ba, Mg or Zn.
U.S. Pat. No. 4,272,296 discloses bismuth-vanadate based yellow pigments diluted with 10-90% by weight of orthorhombic BaSO.sub.4.
U.S. Pat. No. 4,316,746 discloses molybdenum- or tungsten-containing, bismuth-vanadate yellow pigments represented by the formula EQU Bi.sub.(1-x/3) M.sub.x V.sub.1-x O.sub.4
wherein M is Mo or W, x varies from 0.075 to 0.317 when M is Mo and from 0.059-0.265 when M is W. The reference indicates that these pigments may also contain a crystalline phase consisting of orthorhombic BaSO.sub.4.
U.S. Pat. No. 4,455,174 discloses a bismuth-vanadate yellow pigment represented by the formula EQU BiVO.sub.4.xBi.sub.2 MoO.sub.6.yBi.sub.2 WO.sub.6
wherein x is 0.6-2.25 and y is 0-0.1. These pigments are prepared by a process in which a solution containing a bismuth (III) salt, a vanadate and a molybdate is acidified until the Ph is less than zero. The pH of the acidic solution is then brought to 0.1-3.5 at 20.degree.-95.degree. C. by means of an alkaline solution which may or may not contain a dissolved tungsten (VI) compound. The precipitated product and the reaction solution are left at 50.degree.-100.degree. C. for 30-120 minutes, and then separated from one another. The product is washed, dried, if required, and heated at 300.degree.-800.degree. C. Example 2 discloses the preparation of a pigment represented by the formula BiVO.sub.4.0.2Bi.sub.2 MoO.sub.6 which corresponds to Bi.sub.7 V.sub.5 MoO.sub.26. German Offenlegungsschrift 3135281, which is the priority document upon which U.S. Pat. No. 4,455,174 is based, indicates that in the above formula, both x and y have values in the range of 0-3 and the sum of x and y is in the range of 0.1-3.
U.S. Pat. No. 4,752,460 discloses bismuth/vanadate/molybdate and bismuth/vanadate/tungstenate pigments represented by the formula EQU (Bi,A)(V,D)O.sub.4
wherein: A is an alkaline earth metal or zinc or mixture thereof; D is Mo, W or mixture thereof; the molar ratio of A:Bi is in the range of 0.1-0.4; and the molar ratio of D:V is in the range of 0-0.4. The notation (Bi,A) means that the bismuth is present in the form of the bismuth (III) ion and is partly replaced by the divalent metal cation A. Vanadium is present as the vanadium (V) ion in the form of the vanadate ion and can be partly replaced by the hexavalent metal cation D as molybdate or tungstenate or mixtures thereof. The reference indicates that these pigments are in the form of tetragonal, scheelitelike crystal structures, and can be coated with an inorganic protective coating such as a silicon compound and a texture-improving agent such as wax.
R. A. Armstrong et al, "Bismuth Titanate Solid Solutions", Mat. Res. Bull. Vol. 7, pp. 1025-1034 (1972) indicates that ferro-electric Bi.sub.4 Ti.sub.3 O.sub.13 can be described as a sequence of alternating EQU (Bi.sub.2 O.sub.2).sup.2+ and (Bi.sub.2 Ti.sub.3 O.sub.10).sup.2-
layers stacked along a common axis. The Bi.sub.2 Ti.sub.3 O.sub.12 units possess perovskitelike structures with corner-linked TiO.sub.6 octahedra surrounding twelve coordinated bismuth ions. Bismuth oxide layers similar in structure to lead oxide separate the perovskite layers. The reference indicates that several di- and trivalent ions substitute readily for bismuth in the perovskite layer, but the octahedral site and the bismuth oxide layer are far less flexible, tolerating only very limited solid solution.
G. N. Subbanna et al, "Super Structures Exhibited by Oxides of the Aurivillius Family, (Bi.sub.2 O.sub.2).sup.2+ (A.sub.n-1 B.sub.n O.sub.3n+1).sup.2-", Mat. Res. Bull., Vol. 22, pp. 205-209 (1987), discloses that Bi.sub.5 Ti.sub.3 FeO.sub.15 and Bi.sub.7 Ti.sub.3 Fe.sub.3 O.sub.21, which are n=4 and n=6 members of the family of oxides of the general formula EQU (Bi.sub.2 O.sub.2).sup.2+ (A.sub.n-1 B.sub.n O.sub.3n+1).sup.2-
consist of (A.sub.n-1 B.sub.n O.sub.3n+1).sup.2+ perovskite layers located between two (Bi.sub.2 O.sub.2).sup.2+ layers. The reference indicates that these oxides show unusual super structures, possibly due to cation ordering.